Coherent light generators – Particular active media – Gas
Reexamination Certificate
2000-09-26
2002-12-17
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Gas
Reexamination Certificate
active
06496527
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an ultraviolet laser apparatus that seals a gas for ultraviolet laser in a chamber and excites the gas for ultraviolet laser by oscillating pulses in the chamber to oscillate a pulsed laser, and to a gas for ultraviolet laser, and particularly to an ultraviolet laser apparatus and a gas for ultraviolet laser to which xenon gas is added to improve laser output in terms of the burst and spike phenomena.
2. Description of the Related Art
Conventionally, in aligners, which use an excimer laser device as a light source, IC chips are exposed on a semiconductor wafer by alternately repeating the exposure and the movement of a stage. To that end, conventional ultraviolet laser apparatuses perform burst operations which repeat a consecutive pulse oscillation operation that consecutively oscillates a prescribed number of laser beam pulses and an oscillation stop for stopping the pulse oscillation for a prescribed time interval.
FIG.
6
(
a
) is a diagram that plots the relationship between energy and burst number when burst operations are conducted with a conventional excimer laser apparatus. As plotted in this diagram, in burst operations with the excimer laser apparatus, the initial energy is high, and thereafter the energy gradually declines. This characteristic is hereinafter called the “burst characteristic.”
FIG.
6
(
b
) is a diagram that plots the relationship between pulses and energy in each burst. As plotted in this diagram, initially in the consecutive pulse oscillation operation, comparatively high energy is obtained, and thereafter the pulse energy gradually declines. This characteristic is hereinafter called the “spike characteristic.”
Thus, when burst operations are conducted using a conventional excimer laser apparatus, such a burst characteristic and spike characteristic will ordinarily occur.
However, if a burst characteristic appears in the laser output oscillated by an excimer laser apparatus, a problem arises in that the exposure amount varies due to fluctuation in the energy of each burst.
Furthermore, if a spike characteristic appears in such laser output, the exposure amount precision will decline further, which is problematic because of the necessity then of conducting complex discharge voltage control.
In other words, conventionally, complex discharge voltage control has been necessitated because of measures taken to prevent initial energy rise due to the spike phenomenon, varying the discharge voltage pulse by pulse, so as to make the discharge voltage (charging voltage) small in the first pulse of the consecutive pulse oscillation in the burst mode, and then make the discharge voltage gradually larger in pulses thereafter.
In view of these facts, an extremely important issue is that of how to efficiently cancel out burst and spike characteristics in laser outputs when an ultraviolet laser apparatus is operated in burst mode.
Art for adding xenon gas into elemental neon gas is disclosed in “Transmission Properties of Spark Preionization Radiation in Rare-Gas Halide Laser Gas Mixes” appearing in IEEE JOURNAL OF ELECTRONICS, Vol. 31, No. 12, December, 1995, pp 2195-2207. However, this conventional technology is nothing more than the technology for making spark pre-ionization intensity large, and is not the technology for canceling burst and spike characteristics in ultraviolet laser output.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to efficiently improve ultraviolet laser outputs in terms of burst and spike characteristics, and to make it possible to enhance and stabilize laser outputs.
To achieve the object stated above, an invention relating to a first aspect of the invention is an ultraviolet laser apparatus for sealing a gas for ultraviolet laser in a chamber, and exciting that gas for ultraviolet laser by effecting pulse discharges in that chamber to oscillate a pulsed laser, wherein a prescribed quantity of xenon gas at a prescribed concentration is supplied to the gas for ultraviolet laser in the chamber, and the burst phenomenon and spike phenomenon appearing in the ultraviolet laser output are reduced.
Thus, with the first aspect of the invention, a prescribed quantity of xenon gas at a prescribed concentration is supplied to the gas for ultraviolet laser in the chamber, and the burst and spike phenomena appearing in the ultraviolet laser output are cancelled out, wherefore the ultraviolet laser output can be enhanced, and that output stabilized, simply, without recourse to complex control.
An invention relating to a second aspect of the invention comprises a xenon gas cylinder wherein is sealed xenon gas for supply to the inside of the chamber, detection means for detecting the concentration of the xenon gas added to the gas for ultraviolet laser inside the chamber, and control means for controlling the quantity of the xenon gas sealed in the xenon gas cylinder that is supplied to the chamber, based on the xenon gas concentration detected by the detection means.
Thus, with the second aspect of the invention, provision is made for detecting the concentration of the xenon gas added to the gas for ultraviolet laser inside the chamber, and for controlling the quantity of xenon gas sealed in the xenon gas cylinder that is supplied to the chamber, wherefore, by merely providing a conventional ultraviolet laser apparatus with a xenon gas cylinder, detection means, and control means, the ultraviolet laser output can easily be enhanced and that output stabilized.
An invention relating to a third aspect of the invention is a gas for ultraviolet laser used in an ultraviolet laser apparatus that excites gas for ultraviolet laser sealed inside a chamber to oscillate a pulsed laser, which gas for ultraviolet laser at least contains xenon gas of a prescribed concentration.
Thus, with the third aspect of the invention, the gas for ultraviolet laser is configured so as to at least contain, besides a halogen gas, xenon gas of a prescribed concentration. Therefore, merely by supplying this gas for ultraviolet laser to the inside of the chamber, the ultraviolet laser output can easily be enhanced and that output can also be stabilized.
An invention relating to a fourth aspect of the invention is the gas for ultraviolet laser described above, containing xenon gas at 200 ppm or lower.
An invention relating to a fifth aspect of the invention comprises xenon gas adsorption means for causing xenon gas to be adsorbed in the chamber wherein xenon gas has not been adsorbed on the surface of the walls therein, and verification means for verifying that the concentration of xenon gas inside a chamber reaches a prescribed concentration when the xenon gas absorption means have caused xenon gas to be adsorbed on the surface of the walls inside that chamber, and xenon gas has been supplied in such quantity that the concentration of the xenon gas inside the chamber will reach the prescribed concentration.
Thus, with the fifth aspect of the invention, sufficient xenon gas is caused to be adsorbed on the surface of the walls inside the chamber, beforehand, in cases where use is made of a chamber having no xenon gas adhering to the surface of the walls therein, such as a chamber newly fabricated and assembled, for example, or reassembled after maintenance treatments such as decomposition and cleaning after being used for a laser. Following thereupon, when xenon gas is supplied inside the chamber in sufficient quantity that the prescribed concentration will be attained, this xenon gas is added to the laser gas without adsorption to the surface of the walls inside the chamber. Hence the concentration of the xenon gas inside the chamber will reach the prescribed concentration. As a consequence, even if a newly assembled or reassembled chamber is used, the concentration of the xenon gas at the initial stage of use will reach the prescribed concentration, and, when conducting burst operations, the burst phenomenon and spike phenomenon appearing in the ultraviolet laser output can b
Sumitani Akira
Terashima Katsutomo
Wakabayashi Osamu
Ip Paul
Komatsu Ltd.
Varndell & Varndell PLLC
Zahn Jeffrey N
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